1. Field of the Invention
The present invention relates to a microswitch and a method for manufacturing the same. In particular, it relates to a microswitch useful as, for example, compact high-frequency switches for cellular phones and other portable communications equipment.
2. Description of the Related Art
In recent years, as the market for portable communications equipment and other various high-frequency equipment has been expanded and the development thereof has been expedited, the production of high-frequency electronic components to be incorporated into these equipment has been stepped up. In particular, with respect to high-frequency signal transmission and reception circuits incorporated into portable communications equipment, requirements for higher performances of compact high-frequency switches used for switching input and output signals of antennas have become even more intensified.
In particular, a high performance is required for achieving a lower insertion loss of high-frequency signal on switch-on and higher isolation on switch-off. Now, in general, the compact high-frequency switch is of a mechanical type by the use of a relay or an electronic type by the use of a PIN diode, or the like. Recently, the use of a microswitch has been studied in accordance with technological development of Micro Electro Mechanical System primarily based on a silicon process (Si-MEMS) to which a semiconductor manufacturing process is applied. Examples of advantages of the microswitch are low the insertion loss of high-frequency signal, relatively low production cost, and low power consumption, as compared with those of other electronic switches.
Examples of driving systems of the microswitch to which the above-described Si-MEMS technology is applied (hereafter referred to as “Si-MEMS switch”) include an electrostatic type in which an electrostatic force serves as a driving force, a thermal type in which a component made of a bimetal is deformed by heating, a magnetic type in which a magnetic force serves as a driving force, and a piezoelectric type in which a driving force of a piezoelectric/electrostrictive element is used. Among them, the microswitch adopting the piezoelectric type driving system is noted since switching of high-frequency signals can be efficiently performed at a low power consumption as compared with those of microswitches adopting other driving systems (refer to JP-K-2003-519821, for example).
However, a Si-MEMS switch proposed in the above-described JP-k-2003-519821 and the like is to switch high-frequency signals through elastic deformation of a Si substrate and, therefore, in some cases, the reliability of long term driving is unsatisfactory. Furthermore, since a piezoelectric film and electrode films are formed on the Si substrate by a vacuum film forming process, occurrence of crack or peeling off of the films resulting from differences in thermal expansion between the Si substrate, the electrode film and the piezoelectric film must be avoided. Therefore, there is a problem in that it is difficult to increase the film thicknesses of the piezoelectric film and the electrode films.
On the other hand, a micro machine switch has been proposed as a microswitch other than the Si-MEMS switch. The micro machine switch is provided with a signal line conductor, a cantilever driven short circuit mechanism made of Cr or the like that is a material having spring characteristics, and a piezoelectric, and the short circuit mechanism is driven by the piezoelectric to displace and short-circuit the signal line conductor and a predetermined grounding conductor, so that a high-frequency signal passing through the signal line conductor is interrupted (refer to JP-A-2003-217421, for example). In addition, a piezoelectric actuator has been proposed, in which a predetermined shape of metal plate with a piezoelectric element stuck on the surface thereof is configured to be driven by bending this piezoelectric element (refer to JP-A-2001-68751, for example). Examples of methods for disposing the piezoelectric (piezoelectric element) on the surface of the metal substrate include a vacuum film forming process and a method in which a piezoelectric element prepared separately is stuck with an adhesive or the like.
However, as described above, there is a problem in that it is difficult to increase the film thicknesses of the piezoelectric element (the piezoelectric film and the electrode films) by the vacuum film forming process. In the case where the piezoelectric element is stuck to the substrate by using the adhesive, the durability of the adhesion surface and the reliability is not always satisfactory. In particular, a microswitch required to reliably switch high-frequency signals for a long term must be further improved.